The present invention relates generally to amplifier circuits and specifically to a voltage switching amplifier with reduced distortion.
Amplifier circuits with a voltage-switching system that obtain higher efficiency than a class-B power amplifier circuit are known and an example of a push-pull amplifier circuit is shown in FIG. 1. It is comprised of an output transistor 1 (1') (components relating to the positive signals are unprimed and components relating to the negative signals are primed and generally in parenthesis) that is connected to a low-voltage power source +V.sub.L (-V.sub.L) and drives the load. Another output transistor 2 (2') that is connected to a high-voltage power source +V.sub.H (-V.sub.H) also drives the load. A comparator, which is not shown in the diagram, compares the input signal voltage with the preset switching voltage (noted as comparison voltage hereafter). A switching device 3 (3') that turns ON with the output of the comparator when the input signal voltage is lower than the comparison voltage conducts the input signal to the output transistor 1 (1'). A switching device 4 (4') that turns ON with the output of the comparator when the input voltage exceeds the comparison voltage conducts the input signal to the output transistor 2 (2'). Input driver circuit 5 supplies the input signal V.sub.i.
When a conventional amplifier circuit with a voltage-switching system configured as in FIG. 1 is used, the output transistor is switched by the magnitude of the input signal voltage, allowing a higher efficiency, as shown by curve a in FIG. 2, than the efficiency of a conventional class-B power amplifier circuit as shown by curve b in FIG. 2.
However, when the above-mentioned conventional amplifier circuit is used, switching distortion occurs in switching from the output transistor connected to the low voltage power source to the output transistor connected to the high voltage power source, resulting in signal distortion. Furthermore, although the distortion factor does not deteriorate too much at low frequency because of the availability of sufficient negative feedback, a shortcoming exists at high frequency when the amount of negative feedback is reduced due to phase compensation, etc. making it difficult to improve the distortion factor.